There is a conventionally known exhaust emission control system used for an exhaust system of an internal combustion engine in which an air/fuel mixture is controlled to have a theoretical air-fuel ratio, the exhaust emission control system employing a three-way catalyst comprising noble metals such as Pt, Rh, and Pd, that is, a noble metal-based three-way catalyst.
In the noble metal-based three-way catalyst, a range of air-fuel ratio enabling a high exhaust emission control rate to be obtained, namely, an A/F window is wide. Therefore, an exhaust emission control system using a noble metal-based three-way catalyst has an advantage that the control of an air-fuel ratio for increasing the exhaust emission control rate is relatively easy to carry out. However, the exhaust emission control system suffers from a problem that the manufacture cost is high due to the use of the noble metal-based three-way catalyst.